Method and apparatus for inspecting direct liquid fuel cell generator, and direct liquid fuel cell generator

ABSTRACT

The invention provides a method for inspecting a fuel cell that can simply inspect fuel cell characteristics.  
     The method is an inspecting method for a direct methanol fuel cell generator comprising an anode electrode including an node catalyst layer, a cathode electrode including a cathode catalyst layer, and N pieces of cells having an electrolyte disposed between the anode electrode and the cathode electrode, for power generation by feeding an aqueous methanol solution to the anode electrode and an oxidant gas to the cathode electrode. The fuel cell generator is inspected by measuring voltage changes of the voltage V of one electromotive unit caused by generating a current density change ΔI or −ΔI (mA/cm 2 ) satisfying the condition of 0.2≦ΔI≦5 in a finite current density I (mA/cm 2 ) loaded on the plural electromotive units arbitrarily connected in series in the fuel cell generator under power generation during a time interval Δt (sec) satisfying the condition of 10 −5 ≦Δt≦0.5.

CROSSREFERENCE TO RELATED APLICATION

[0001] This application is based upon and claims the benefit of priorityfrom the prior Japanese Patent Application No.2002-113323, filed on Apr.16, 2002; the entire contents of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The invention relates to an inspection method for a direct liquidfuel cell generator that generates electricity by supplying a liquidfuel and an oxidant, an inspection apparatus for a direct liquid fuelcell generator using the inspection method, and a direct liquid fuelcell generator comprising the inspection apparatus of the fuel cellgenerator.

[0004] 2. Description of the Related Art

[0005] Fuel cells are generators that directly convert a chemical energy(free energy of combustion reactions) into a direct electrical energy,and are expected to have a higher conversion efficiency than thermalpower generation. While power generation efficiency decreases when thescale of thermal power generation is small, power generation by the fuelcell is not decreased in small scale operation. Accordingly, the fuelcell is suitable for small scale power generation.

[0006] Among the fuel cells, developments of solid polymer electrolytefuel cells have been accelerated in recent years as automobile powersources and domestic power sources. A gas containing hydrogen isintroduced into an anode side, and oxygen gas or air is introduced intoa cathode side in this solid polymer electrolyte fuel cell. Anelectromotive force is generated by the following reactions representedby the chemical formulae 1 and 2 at the anode and cathode sides,respectively.

Anode: 2H₂→4H⁺+4e ⁻  chemical formula 1

Cathode: O₂+4H⁺+4e ⁻→2H₂O   chemical formula 2

[0007] The equations mean that electrons and protons are formed fromhydrogen by means of a catalyst within the anode. The electrons aretaken out of the cell through external circuits, and are used for powergeneration. The protons move in the solid electrolyte membrane andarrive at the cathode, and water is generated by a reaction between theelectrons, oxygen, and the protons by a catalyst within the cathode.Electric power is generated by such cell reaction.

[0008] On the other hand, a direct methanol fuel cell has been noticedin recent years. FIG. 1 shows the structure of the direct methanol fuelcell. In the direct methanol fuel cell, a proton conductive electrolytemembrane (a perfluorocarbon sulfonic acid ion exchange membrane; Nafionmade by DuPont Co. is preferably used) is sandwiched between an anodeelectrode and a cathode electrode. Each electrode comprises a substrateand a catalyst layer which comprises a catalyst and a proton conductiveelectrolyte. The catalyst is usually a precious metal or an alloy of theprecious metal, which is used by being supported on carbon black. Thecatalyst is not supported on carbon black in some cases. A Pt—Ru alloyis preferably used as catalyst at the anode side, while Pt is preferablyused as catalyst at the cathode side. Methanol and water are introducedinto the anode side, and oxygen gas or air is introduced into thecathode side for operation. The reactions represented by the followingchemical formulae 3 and 4 occur at the anode and cathode sides,respectively.

Anode: CH₃OH+H₂O→CO₂+6H⁺+6e ⁻  chemical formula 3

Cathode: (3/2) O₂+6H⁺+6e ⁻→3 H₂O   chemical formula 4

[0009] These equations mean that electrons, protons and carbon dioxideare formed by the catalyst in the anode catalyst layer. Carbon dioxidegenerated is exhausted in the atmosphere. The electrons are taken out ofthe fuel cell through an external circuit, and are used for powergeneration. The protons move in a proton conductive electrolytemembrane, and arrive at the cathode. Water is formed in the cathodecatalyst layer by a reaction of the electrons and oxygen and protons.The operating temperature of this direct methanol fuel cell is usually50 to 120° C.

[0010] It was a drawback that a reformer should be provided in the fuelcell system and the entire system is forced to be large size, when a gascontaining hydrogen is used as a fuel as in the solid polymerelectrolyte fuel cell as described above, since the hydrogen gas isgenerally obtained by reforming methanol, natural gas or gasoline. Thereforming process is generally performed at a high temperature of 250 to300° C. In contrast, the system itself may be compact in the directmethanol fuel cell since no reformer is needed, and the power generationprocess can proceed at a relatively low temperature. Accordingly, thedirect methanol fuel cell has been developed in recent years forapplying it to a portable power source and an electric car power sourceby taking notice of this advantage of the direct methanol fuel cell.

[0011] An aqueous methanol solution or a vaporized mixture of methanoland water is supplied for feeding methanol and water to the fuel cell inthe direct methanol fuel cell generator. Since a vaporizer should beprovided as an auxiliary equipment of the fuel cell when methanol andwater are supplied as an evaporated mixed gas, the total fuel cellsystem inevitably becomes large size. On the contrary, the system may besmall size when the aqueous methanol solution is supplied, since novaporizer is needed.

[0012] However, there are many difficult problems in the direct methanolfuel cell as described above as compared with solid polymer electrolytefuel cells.

[0013] As an problem, the fuel supplied to the electrode moves withinthe electrode, enters a proton conductive electrolyte, moves within theelectrolyte to arrive at the catalyst, and is used for generatingelectric power. The proton conductive electrolyte exhibits protonconductivity by being impregnated with water. It has been elucidated inthe foregoing studies that introduction of methanol reduces protonconductivity (for example T. J. Chou and A. Tanioka, J. Phys. Chem.,B102 (1998), 129). Methanol, water and oxygen as fuel components move bybeing dissolved into impregnated water in the proton conductiveelectrolyte in the catalyst layer. Reduced proton conductivity alsoreduces diffusion of water that moves by being pulled with the protons.Consequently, mobility of water and diffusion of methanol that iscompletely mixed with water are also decreased at the anode electrode.Methanol also exists at the cathode electrode since methanol supplied tothe anode electrode arrives at the cathode electrode through the protonconductive electrolyte membrane. Accordingly, diffusion of water alsodecreases in the proton conductive electrolyte within the cathodecatalyst layer. As a result, diffusion of oxygen is reduced since oxygendiffuses within the electrode by being dissolved in water in the protonconductive electrolyte. In summary, the fuel cell is confronted with asevere problem that diffusion abilities of all the fuel components ofmethanol, water and oxygen as fuels are reduced. Accordingly, it isinevitable for practical applications to elucidate characteristic valuesthat can be readily measured in close relation with the degree ofdiffusion of the fuel.

[0014] As another problem, diffusion of the fuel is so poor immediatelyafter resumption of operation that equipment is operated under acondition where response to variation of load is very poor, since waterimpregnated in the proton conductive electrolyte is dried up duringpause period of the operation of the direct methanol fuel cellgenerator. In case of generators, particularly generators for potableappliances and automobiles in which the fuel cell is intermittentlyoperated in daily work and variation of load occurs frequently, theresponse to variation of load becomes very poor, thereby causingtroubles in driving the appliances. Consequently, the trouble may inducesevere accidents that may threaten human life. Therefore, it should beconfirmed how is the response to variations of load, and how much is theperformance of the fuel cell before and during power generation.

[0015] As a different problem, the perfluorocarbon sulfonic acidmembrane is swelled by being impregnated with water, and swelling ismuch larger when the membrane is impregnated with methanol. Therefore,the proton conductive electrolyte membrane and catalyst layer aredamaged by excessive swelling when a high concentration of aqueousmethanol solution is supplied by some reasons, and the performance ofthe fuel cell is largely decreased.

[0016] Consequently, it has been recognized that development of a simplemethod for deciding the performance of the fuel cell during powergeneration is also important.

[0017] The fuel cell may be severely damaged by the condition of theproton conductive electrolyte membrane, or by the condition of fueldiffusion, in the direct methanol fuel cell generator as describedabove. Therefore, an inspection method for always verifying thecondition of the fuel cell is important.

[0018] The characteristics of the fuel cell have been mostly evaluatedby measuring an I-V curve. However, the measurements of the I-V curvealso involves the results including other lines of information such ascatalyst activity and internal resistance other than the degree ofdiffusion of the fuel. Furthermore, voltages should be measured in awide range of current density for measuring the I-V curve. Inparticular, a stationary state operation should be naturally interruptedfor measuring the I-V curve of the fuel cell that is under a long-runstationary operation. Since this inspection procedure costs much labor,it cannot be readily applied for evaluation and inspection.

SUMMARY OF THE INVENTION

[0019] Accordingly, it is an object of the present invention to providea method for inspecting a direct methanol fuel cell generator that isable to readily obtain a line of information closely related to thedegree of diffusion of the fuel during power generation of the fuelcell, and to exactly decide the performance of the fuel cell.

[0020] It is another object of the invention to provide a simple andhighly accurate inspection apparatus for realizing the inspectionmethod, and a direct methanol fuel cell generator comprising theinspection apparatus.

[0021] According to a first aspect of the present invention, there isprovided An inspection method for a direct liquid fuel cell generatorhaving a plurality of cells, each comprising an anode electrodeincluding an anode catalyst layer, a cathode electrode including acathode catalyst layer, and an electrolyte disposed between the anodeelectrode and the cathode electrode, for power generation by supplying aliquid fuel to the anode electrode and an oxidant gas to the cathodeelectrode, wherein validity of the fuel cell is decided based on anobservation result of time dependent changes of the voltage V of oneelectromotive unit which is caused by generating a current densitychange ΔI or −ΔI (ΔI in mA/cm² unit represents a positive value)satisfying the condition of 0.2≦ΔI≦10 in a current density I (mA/cm²),which is taken out from an arbitrary number of cells connected in seriesconstituting the direct liquid fuel cell generator under powergeneration, during a time interval Δt (sec) satisfying the condition of10⁻⁵≦Δt≦0.5.

[0022] According to a second aspect of the present invention, there isprovided An inspection apparatus for a direct liquid fuel cell generatorhaving a plurality of cells, each comprising an anode electrodeincluding an anode catalysis layer, a cathode electrode including acathode catalyst layer, and an electrolyte disposed between the anodeelectrode and the cathode electrode, for power generation by supplying aliquid fuel to the anode electrode and an oxidant gas to the cathodeelectrode, the apparatus comprising: a load connected to an output fromthe direct liquid fuel cell generator to consume output power thereof;means connected to the output from the direct liquid fuel cellgenerator, for changing an output current density by controlling theload; means for measuring an output voltage from the direct liquid fuelcell generator; and a decision device connected to the current densitycontrol means and voltage detection means, for discriminating thecondition of the fuel cell generator from the initiation time of thecurrent density change, and from the measured results of the change ofthe output voltage.

[0023] According to a third aspect of the present invention, there isprovided a direct liquid fuel cell generator having a plurality ofcells, each comprising an anode electrode including an anode catalysislayer, a cathode electrode including a cathode catalyst layer, and anelectrolyte disposed between the anode electrode and the cathodeelectrode, for power generation by supplying a liquid fuel to the anodeelectrode and an oxidant gas to the cathode electrode, the liquid fuelcell generator comprising: a load connected to an output from the directliquid fuel cell generator to consume output power thereof; meansconnected to the output from the direct liquid fuel cell generator, forchanging an output current density by controlling the load; means formeasuring an output voltage from the direct liquid fuel cell generator;and a decision device connected to the current density control means andvoltage detection means, for discriminating the condition of the fuelcell generator from the initiation time of the current density change,and from the measured results of the change of the output voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024]FIG. 1 is a schematic cross section showing the structure of acell as a power generation element of a direct methanol fuel cell;

[0025]FIGS. 2A and 2B are graphs for describing the principle of theinspection method of the invention, where FIG. 2A is a graph showing atime dependent change of the voltage observed by increasing the loadedelectric current at a time of T=0; and FIG. 2B is a graph showing a timedependent change of the voltage observed by decreasing the loadedelectric current at a time of T=0;

[0026]FIG. 3 is a graph showing a time dependent change of the voltagecorresponding to the change of the current density in an example of theinvention when the load current is increased from 0 mA/cm² to 5 mA/cm²,from 5 mA/cm² to 10 mA/cm², and from 10 mA/cm² to 15 mA/cm²;

[0027]FIG. 4 is a graph showing the voltage response by changing thetime when the current density is changed, wherein the graph shows a timedependent changes of the voltage by changing the load current when thelapse of time ΔT is changed to 10⁻⁵, 0.5, and 3 seconds, respectively.

[0028]FIG. 5 is a graph showing the ΔT dependency of T1 in an example ofthe invention;

[0029]FIG. 6 is a graph showing a time dependent change of the voltageobserved by changing the magnitude of ΔI in an example of the invention;

[0030]FIG. 8 is a schematic diagram showing an example of the inspectionapparatus for the direct methanol fuel cell generator of the invention;

[0031]FIG. 9 is a flow chart showing the inspection procedure using theinspection apparatus for the direct methanol fuel cell generator of theinvention;

[0032]FIG. 10 is a graph showing a time dependent change of the voltageof a fuel cell 1 before and after changing the load current;

[0033]FIG. 11 is a graph showing a time dependent change of the voltageof a fuel cell 2 before and after changing the load current;

[0034]FIG. 12 is a graph showing a time dependent change of the voltageof a fuel cell 3 before and after changing the load current;

[0035]FIG. 13 is a graph showing I-V curves of the fuel cells 1, 2 and3, respectively;

[0036]FIG. 14 is a graph showing current density dependency of outputdensities of the fuel cells 1, 2 and 3, respectively;

[0037]FIG. 15 is a schematic diagram showing another example of theinspection apparatus for the direct methanol fuel cell generator of theinvention;

[0038]FIG. 16 is a flow chart showing the inspection procedure using theinspection apparatus for the direct methanol fuel cell generator of theinvention;

[0039]FIG. 17 is a graph showing the time dependent changes of thevoltages of fuel cells 6 and 8, respectively, before and after changingthe load current;

[0040]FIG. 18 is a graph showing the I-V curves in another example ofthe invention; and

[0041]FIG. 19 is a graph showing the current density dependencies of theoutput current density in another example of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0042] The invention will be described in detail with reference to theembodiments.

[0043] [Fuel Cell]

[0044] An example of a fuel cell suitable for applying the invention isshown in FIG. 1. The cell comprises an anode substrate 1, an anodeelectrode 3 including an anode catalyst layer 2, a cathode catalystlayer 4, a cathode electrode 6 including a cathode substrate 5, and aproton conductive electrolyte membrane 7 disposed between the anodeelectrode 3 and the cathode electrode 6. A direct methanol fuel cellcomprises a plurality of cells, which is shown in FIG. 1. Terminals (notshown) are connected to the respective electrodes, and devices forsupplying a liquid fuel and an oxidant gas to the anode electrode andcathode electrode, respectively, are provided in the fuel cell. Aninspection apparatus and external load to be described below areconnected to respective terminals.

[0045] While the fuel cell suitable for applying the invention comprisesthe plural cells, a fuel feed portion including a fuel tank forsupplying a liquid fuel to the cells, an oxidant feed portion, and powerterminals of the fuel cell generator, a cell having another structureand material except those shown in FIG. 1 may be employed in the fuelcell.

[0046] Examples of the liquid fuel used for the fuel cell of theinvention include aqueous solutions of organic compounds such asmethanol, ethanol, formic acid, formaldehyde and dimethyl ether. Amongthese compounds, methanol, formic acid, formaldehyde and dimethyl etherare preferable due to their high reactivity, and methanol is mostpreferable since it is efficiently allowed to react by aplatinum-ruthenium catalyst.

[0047] [Inspection Method]

[0048]FIGS. 2A and 2B are graphs for describing the principle of theinspection method of the invention.

[0049]FIG. 2A is a graph showing changes of the voltage V dependent onthe time T caused by a change ΔI of in the current density flowingthrough a load connected to the fuel cell. The voltage rapidly decreasesimmediately after the change ΔI of the current density flowing in theload. This current change becomes gentle in accordance with the lapse oftime, and the voltage becomes minimum at T=T1. The voltage monotonouslyincreases at T>T1, and settles at a constant level in accordance withthe further lapse of time.

[0050] This phenomenon is conjectured to arise by the following reasons.Since the current density has increased at T=0, the surface of thecatalyst and the neighboring area thereof becomes locally deficient inthe fuel. Consequently, the voltage decreases with the lapse of time dueto diffusion polarization. On the other hand, the fuel that has beendeficient is supplied to the catalyst and the neighboring area thereofto ameliorate deficiency of the fuel. Accordingly, the voltage turns toincrease with the lapse of time at T=T1 and thereafter, and settles at aconstant level.

[0051]FIG. 2B is a graph showing a change −ΔI of the current densityflowing through the load connected to the fuel cell, or the timedependent change of the voltage V caused by the decreased currentdensity. The voltage rapidly increases immediately after a currentdensity change −ΔI. Then, the voltage change becomes gentles with thelapse of time, and the voltage becomes maximum at T=T1. The voltagemonotonously decreases at T>T1, and settles to a constant level with thefurther lapse of time.

[0052] This phenomenon is conjectured to arise by the following reasons.Since the current density has decreased at T=0, the fuel becomes excesson the surface of the catalyst in the electrode and at the neighboringarea thereof as compared before. Consequently, diffusion polarizationreduces to increase the voltage with the lapse of time. On the otherhand, the excess fuel moves to the catalyst and the neighboring areathereof to ameliorate local excess of the fuel. Accordingly, the voltagedecreases with the lapse of tine at T=T1 and thereafter, and settles toa constant level.

[0053] Since the change of the electromotive force accompanied by thechange of the current density shown in FIGS. 2A and 2B represents thedegree of diffusion of the fuel as will be apparent from thedescriptions above, the fuel cell may be readily inspected based on theprinciple above. Diffusion of the fuel is shown to be quite poor whenthe time T1 that shows the time when the voltage generated by thecurrent change shows a minimum or maximum is larger than a prescribedtime. This phenomenon indicates that the electromotive force of the fuelcell cannot follow the change of load during the operation of the cell.The fact that the time showing the maximum or minimum voltage is out ofa prescribed range when the voltage change caused by the current densityis measured clearly indicate that the performance of the fuel cell ispoor. Therefore, the fuel cell can be simply and objectively inspectedbased on a criterion that T1 is within a prescribed time.

[0054] When Δt that is a time for allowing the current density to changeis too long, the behaviors shown in FIGS. 2A and 2B become so dull thata precise inspection becomes unable in the invention. On the other hand,making Δt too short is not practically preferable, since the structureof the inspection apparatus becomes complex and the apparatus becomesexpansive. A range of the time Δt for allowing the current density tochange of 10⁻⁵≦Δt≦0.5 is preferable, because the changes as shown inFIGS. 2A and 2B becomes evident to an extent enough for achievingsufficient inspection provided by the invention, and because theinspection apparatus is cheaply manufactured. A range of 10⁻⁵≦Δt≦2×10⁻³is more preferable, since the change is more evidently observed.

[0055] The inspection becomes difficult due to a too small voltagechange generated when ΔI that is the current density change is toosmall. On the other hand, too large ΔI is also not practicallypreferable, since the fuel is wasted for unnecessary power generation ofthe fuel cell generator, or the power generation state is excessivelydisturbed. Therefore, the practically available range is 0.2≦ΔI≦10, morepreferably 0.2≦ΔI≦5. A range of 0.2≦ΔI≦2 is particularly preferable,because disturbance of the fuel cell generator under power generation isvery small.

[0056] While the current density change is linear in FIGS. 2A and 2B,the current density change may be different patterns. For example, thechange may be a curve or a gradation such as two steps or more ofchanges. Alternatively, the change may be not necessarily a monotonousincrease or decrease.

[0057] The inspection method of the invention should be performed in theprocess when the current density I generated by the fuel cell generatoris a non-zero finite value. As an example, FIG. 3 is a graph showingtime dependent changes of the voltage corresponding to the change of thecurrent density when the current densities flowing in the load connectedto the fuel cell are increased from 0 mA/cm² to 5 mA/cm², from 5 mA/cm²to 10 mA/cm², and from 10 mA/cm² to 15 mA/cm², respectively. The time T1that shows the minimum level of the output voltage of the fuel cellcaused by the current density change is in the range of 4 to 5 seconds,when the current density is increased from 5 mA/cm² to 10 mA/cm², andfrom 10 mA/cm² to 15 mA/cm². T1 is prolonged to a far longer time of 78seconds when the current density is increased from 0 mA/cm² to 5 mA/cm².

[0058] It was revealed from repeated experiments by the inventors that afar more longer voltage maximum or minimum time T1 is observed when acondition that gives a current density of 0 mA/cm² has appeared at leastonce or more in the process of change of the current density, ascompared with the case where no such condition that gives a currentdensity of zero has not been experienced in the process. The inspectionmethod provided by the invention cannot be used when the minimum ormaximum time of the voltage change is long, since the inspectionefficiency is decreased to compromise reliability to the inspectionresults.

[0059] In the method that has been conventionally used for inspectionand evaluation of the fuel cell generator, the current density generatedby connecting the fuel cell to the load is changed to zero for a quiteshort period of time (usually several microseconds), and a currenthaving the same magnitude as that immediately before reducing thecurrent to zero is again loaded thereafter. The fuel cell is inspectedand evaluated by the voltage change observed between before and afterreducing the current density to. zero (a current shut-down method). Thismethod is naturally quite different from the method provided by theinvention, because the process of change of the current density onceexperiences zero current in the conventional method, and a highlyreliable inspection is impossible due to disturbance of the inspectionresults as described above.

[0060] Since the length of the time T1 when the voltage as a criterionof validity of the fuel cell is minimum or maximum changes depending onthe characteristics required for the fuel cell inspected, the structureof the electrolyte catalyst layer and electrolyte membrane, thecompositions of the catalyst and electrolyte membrane, the electrodearea, the flow rate of the fuel, the structure of the flow passagewayplate of the fuel, the operation temperature, and I and ΔI, it may beappropriately determined by taking these conditions into consideration.However, it is quite preferable that the setting time of T1 is shorterthan 15 seconds according to the experiments by the inventors. Since theperformance of the fuel cell is markedly decreased when T1 exceeds 15seconds, it is difficult to practically use such fuel cell.

[0061] [Inspection Apparatus: First Inspection Apparatus]

[0062] The inspection apparatus 17 shown in FIG. 8 comprises an externalload 18 connected to the output from the fuel cell 11, a voltagedetector 14 that measures the output voltage of the fuel cell 11, adecision device 15 for deciding by importing the voltage change measuredby the voltage detector 14, and an indicator 16 for indicating thedecision results of the decision device 15.

[0063] The external load 18 is provided for consuming the output powerof the fuel cell in the inspection apparatus 17 while controlling theamount of load based on the control signal from the decision device 15.A commercially available electronic load apparatus (for example acombination of EML-150L load module and EML-30B frame made by FujitsuAccess Ltd.) may be actually used as the load.

[0064] The voltage detector 14 is provided for converting the voltage ofthe power exported from the fuel cell 11 into a form capable of signalprocessing, and an apparatus for exporting the applied voltage intodigital signals by an analogue-digital converter.

[0065] The decision device 15 is provided for changing the currentdensity of the output power from the fuel cell 11 with a given timeperiod and given magnitude, thereby changing the output voltage of thefuel cell 11 while permitting real time input of the output voltagelevel from the voltage detector 14. Validity of the performance of thefuel cell as an object of the inspection is decided based on the changeabove. This device may be realized using a one-chip computer, generaluse microcomputer or logic circuitry.

[0066] The indicator 16 is provided for indicating the results from thedecision device 15 or informing them by means of light, sound orvibration, and examples of the device include a display such as CRT andliquid crystal, a lamp such as LED, and a speaker.

[0067] While the fuel cell as shown in FIG. 18 exports the electricpower through four terminals, the output terminals may be composed oftwo terminals comprising a terminal at the positive electrode and aterminal at the negative electrode. When the external load connected tothe fuel cell has a large capacity for the electric current, voltagedrop of the electric power applied to the voltage detector becomes solarge when the fuel cell comprises two terminals that the sensed resultsare affected by the voltage drop. Accordingly, four terminals arepreferably used in this case.

[0068] The inspection procedure using the inspection apparatus asdescribed above is described below with reference to FIG. 9 as a flowchart of the procedure.

[0069] In FIG. 9, the minimum time T1_(min) indicating the minimum ormaximum voltage as a criterion of validity of the fuel cell as theobject of inspection, the maximum time T1_(max) indicating the minimumor maximum of the voltage, and the current density I and the change ofthe current density ±ΔI are set (S102) after the start of the inspection(S101). T1_(min) and T1_(max) determine the minimum allowable time andmaximum allowable time.

[0070] The fuel cell is started to operate (S103), and a load current isallowed to flow after connecting the load 8 to the fuel cell, and timedependent changes of the voltage applied on the load are recorded(S104). The load current is changed in this state (S105) while applyingthe voltage to the load, and T1 as the time when the voltage indicatesthe minimum or maximum level is determined (S106). T1 is decided whetherit is within a prescribed time interval or not (S107), and the indicatorissues a warning that the fuel cell is defective when T1 is out of theprescribed time interval (S108) to allow the inspection to come to itsend (S109). The indicator indicates that the fuel cell is successful(S110) when no defects are found, and the inspection is completed(S111). Two or more setting ranges of T1 may be provided in order toclassify the inspected fuel cell generator in more detail to moreprecisely discriminate the condition of the fuel cell.

[0071] [Inspection Apparatus: Second Inspection Apparatus]

[0072] Another example of the inspection apparatus of the invention isshown in FIG. 15. The same reference numerals are given to the samemembers in FIG. 15 as those in FIG. 8. Different from the apparatus inFIG. 8, the power output from the fuel cell 11 is dispensed into theexternal load 12 consuming the output and the inspection load 13 forinspection of the invention. The decision device 15 measures the loadgenerated by the external load 12 and inspection load 13 while changingthe load on the fuel cell by controlling the inspection load 13 in orderto observe the change of the output voltage. The inspection apparatus inthis embodiment is possible to discriminate the load of the fuel celland the load for inspection, thereby making it possible to construct afuel cell generator that can be most commonly used.

[0073] As is evident from FIG. 15, an aqueous methanol solution and anoxidant fuel are supplied to the fuel cell 11, and the fuel cell isoperated such that the load current flows through the load 12 connectedto the output power of the fuel cell 11. The inspection device 17 in theembodiment comprises the inspection load 13, voltage detector 14,decision device 15 and indicator 16. The inspection load 13 is used inorder to change the load level that changes the load current flowingfrom the fuel cell.

[0074] While the output pour is also taken out through four terminals inthe fuel cell in FIG. 15, two terminals may be used for this purpose assame in FIG. 8 described above.

[0075] The inspection procedure using the inspection apparatus will bedescribed below with reference to FIG. 16 as a flow chart thereof. Afterthe start (201) of inspection, the load current density I is read out(S202) as shown in FIG. 16. Then, T1 min, T1 max and ±ΔI are set basedon the current density I (S203). However, these values may be setwithout reading out the current density I. Subsequently, the voltage isdetected using the voltage detector 14 to record the time dependentchanges (S204). Then, changes of ±ΔI are given to the load currentdensity using the inspection load (S205), T1 is determined (S206), andT1 is determined whether it is within a prescribed range or not (S207).A warning is issued using an indicator when T1 is out of the prescribedrange (S208), the load is controlled for security or the like (S209),and the inspection comes to its end by deciding that the fuel cell isdefective (S210). Control of the load may be omitted, if it isdesirable. Alternatively, when T1 is within the prescribed range (S211),the inspection comes to its end (S212) based on the decision that thefuel cell is successful without any problems. Two or more setting rangesof T1 may be provided in order to classify the inspected fuel cellgenerator in more detail for controlling the load.

[0076] [Fuel Cell Generator]

[0077] The inspection apparatus is connected to the direct methanol fuelcell in the fuel cell generator of the invention, and the fuel cell isfurther connected to an external load.

[0078] The fuel cell generator of the invention may be housed in ahousing as one power generator, or the fuel cell generator may bedivided into a plurality of members that are electrically ormechanically connected with each other as a power generator system. Itis preferable to integrate the housing as a power generator in order touse it as a power source of portable electronic appliances. The powersource for driving the generator may be supplied from the fuel cellitself. However, since various control devices should be operated forindicating the condition of the fuel cell even when the fuel cell is atrest, another cell is preferably mounted.

[0079] The inspection method and procedure of the fuel cell generatorare preferably executed according to a program written in a nonvolatilememory integrated into the decision device constituting the inspectionapparatus.

EXAMPLE

[0080] While the invention is described in more detail based on theexamples, the invention is not restricted to these examples.

Example 1

[0081] Assembling of the Direct Methanol Fuel Cell

[0082] The following is the method for manufacturing the cell of thefuel cell generator used in the example of the invention. Carbon blackfor supporting the anode catalyst (Pt:Ru=1:1) and carbon black forsupporting the cathode catalyst (Pt) were produced by a method known inthe art (R. Ramakumar et. al., J. Power Sources 69 (1997), 75). Theamounts of the supported catalysts were 30 and 15 parts by weight on theanode and cathode, respectively, relative to 100 parts by weight ofcarbon.

[0083] For preparing the anode electrode, a perfluorocarbon sulfonicacid solution (Nafion solution SE-20092 made by DuPont Co.) and ionexchange water were added to carbon black for supporting the anodecatalyst prepared in the foregoing process, and a paste was prepared bydispersing carbon black for supporting the anode catalyst. This pastewas applied on a sheet of carbon paper TGPG-120 (made by E-TEK Co.)after water repelling treatment followed by drying.

[0084] For preparing the cathode electrode, a perfluorocarbon sulfonicacid solution (Nafion solution SE-20092 made by DuPont Co.) and ionexchange water were added to carbon black for supporting the cathodecatalyst prepared in the foregoing process, and a paste was prepared bydispersing carbon black for supporing the cathode catalyst. This pastewas applied on a sheet of carbon paper TGPG-090 (made by E-TEK Co.)after water repelling treatment followed by drying.

[0085] The cell shown in FIG. 1 was prepared by bonding the anodeelectrode and cathode electrode prepared in the foregoing process onboth faces, respectively, of a commercially available perfluorocarbonsulfonic acid membrane by hot-press (125° C., 5 minutes).

Example 2

[0086] Determination of Inspection Condition

[0087] The fuel cell was assembled by connecting five cells prepared asdescribed above in series, followed by connecting an aqueous methanolfeed device and a oxidant feed device.

[0088] The fuel cell was operated while changing the current density bychanging the time ΔT as a time required for changing the currentdensity. In this experiment, the current density I was set at 145 mA/cm²and the current density difference ΔI was set at 5 mA/cm² to change thecurrent density from 145 mA/cm² to 150 mA/cm². An aqueous methanolsolution with a concentration of 2M was sent to the anode electrodeusing a commercially available feed pump. Air was sent to the cathodeside using a commercially available air pump. The flow rate of air wascontrolled using a commercially available mass flow controller. Thecommercially available electronic load apparatus described above wasused as the load for exporting the power of the fuel cell. Acommercially available digital multimeter was used for sensing thevoltage. The direct methanol fuel cell with an electrode area of 10 cm²was operated by controlling the operation temperature of the fuel cellat 70° C.

[0089] The results of power generation test under the operationcondition above are shown in FIG. 4. In FIG. 4, time dependence of thevoltage due to the current change is shown.

[0090] In FIG. 4, the solid line, broken line and dotted line denote thechanges of the current density at ΔT of 10⁻⁵, 0.5 and 3 seconds,respectively. The result at ΔT of 3 seconds is evidently different fromthe results at ΔT of 10⁻⁵ and 0.5 seconds, and voltage drop and voltageraise thereafter are quite gentle. T1 was 5.3 seconds at ΔT of 10⁻⁵second, 5.5 seconds at ΔT of 0.5 second, and 17 seconds at ΔT of 3seconds.

[0091]FIG. 5 shows ΔT dependency of T1. While T1 is almost constant inthe range of ΔT of 0.5 second or less, T1 is monotonously increased withthe increase of ΔT when it is larger than 0.5 second. These results showthat the voltage change provided in the invention becomes dull when ΔTis larger than 0.5 second, and the method of the invention cannot beused for inspection. Accordingly, it was found that the preferable upperlimit of ΔT is 0.5 second.

Example 3

[0092] Determination of Inspection Condition

[0093] The magnitude of the change of the current density ΔI was changedusing the same fuel cell as in Example 2, and the time T1 beforeattaining the minimum or maximum voltage change was investigated.

[0094] The current density I was set at 170 mA/cm², and a change of −ΔIwas given to the current density. An aqueous methanol solution with aconcentration of 2M was sent to the anode electrode using a commerciallyavailable feed pump. Air was sent to the cathode side using acommercially available air pump. The flow rate of air was controlledusing a commercially available mass flow controller. The commerciallyavailable electronic load apparatus described above was used as the loadfor exporting the power of the fuel cell. A commercially availabledigital multimeter was used for sensing the voltage. The direct methanolfuel cell with an electrode area of 25 cm² was operated by controllingthe operation temperature of the fuel cell at 80° C.

[0095] The results are shown in FIG. 6. In figure, solid line indicatesa case where ΔI is set to 2, and broken line indicates a case where ΔIis set to 0.1. When ΔI is 2, the maximum value of voltage is clear, sothat T1 was determined to be 6.5 seconds. On the other hand, when ΔI is0.1, change in voltage is extremely small, so that the maximum value wasnot discriminated, and it was not possible to clearly determine T1unlike when change ΔI in current density is 2.

[0096]FIG. 7 shows ΔI dependency of T1. It was found that precisedecision of T1 is difficult when ΔI is less than 0.2 due to large errorbars. Accordingly, the preferable lower limit of ΔI was found to be 0.2.

Example 4

[0097] Inspection Apparatus 1

[0098] The example of inspection using the fuel cell generator shown inFIG. 8 will be described below.

[0099] A program operating as a decision device and indication devicewas created using a commercially available programming language operatedon a PC, and was used as the decision device and indication device.

[0100] The following three kinds of cells were prepared by differentpreparation conditions, and the cells were inspected using theinspection apparatus of the invention.

[0101] Cell 1: the fuel cell described in Example 2 was assembled usingthe cell prepared in example 1.

[0102] Cell 2: the cell prepared in Example 1 was immersed in a 4Maqueous methanol solution for 30 hours, and the fuel cell described inExample 2 was assembled.

[0103] Cell 3: the cell prepared in Example 1 was immersed in a 7Maqueous methanol solution for 30 hours, and the fuel cell described inExample 2 was assembled.

[0104] A 2M aqueous methanol solution was sent to the anode side at aflow rate of 0.6 ml/minutes with a commercially available feed pumpusing the cells 1 to 3. Air was sent at a flow rate of 60 ml/minute tothe cathode side using a commercially available air pump. The flow rateof air was controlled using a commercially available mass flowcontroller. A commercially available electronic load apparatus was usedas the load. A commercially available multimeter was used for thevoltage detector. GPIB interface was attached to PC, and the load andinspection load, and voltage detector were connected to the interfaceusing commercially available GPIB cable.

[0105] I and ΔI were set at 30 mA/cm² and 5 mA/cm², respectively, usingthe inspection apparatus, and the current density was changed from 30mA/cm² to 35 mA/cm². Δt was set at 10⁻⁴. It was confirmed that the loadwas changed within 10⁻⁴ second as confirmed with a commerciallyavailable ammeter. T1_(min) was set at 1 second, and T1_(max) was set at5 seconds.

[0106] A commercially available buzzer as an indicator was adjusted sothat it sounds when T1 is out of the prescribed range. Other indicatorsavailable include a buzzer or ring, a LED or lamp, a vibrator, or asmelling device, or a combination thereof. The indicator is not alwaysrequired.

[0107]FIG. 10 shows the time dependent change of the voltage of the cell1 before and after the change of load current. This cell had thesmallest T1 of 2.3 seconds among the three cells. The cell 1 was decidedto be good from this result.

[0108]FIG. 11 shows the time dependent change of the voltage of the cell2 before and after the change of load current. T1 of this cell was 2.3seconds. The cell 2 was decided to be defective from this result.

[0109]FIG. 12 shows the time dependent change of the voltage of the cell3 before and after the change of load current. T1 of this cell was 149.5seconds. The cell 3 was decided to be defective from this result.

[0110]FIG. 13 shows the results of measurements of the I-V curves of thecells 1, 2 and 3, respectively, and the corresponding current densitydependencies of the output current density are shown in FIG. 14. As isevident from the inspection results of the invention, the cell 1 had thehighest performance. The cell 3 showed the worst performance, and thecell 2 showed an intermediate performance. Such differences were causedby the difference of damages suffered by the proton conductiveelectrolyte used for each cell. The cell 3 had suffered the largestdamage among the three cells since it was immersed in the mostconcentrated aqueous methanol solution. The cell 2 had suffered smalldamage since it was immersed in a relatively small concentration of theaqueous methanol solution, and showed better performance than the cell3. The cell 1 showed best performance without suffering from anydamages. It is conjectured that the difference of the degree of damagesis reflected on the mobility of the fuel within the proton conductiveelectrolyte that is related to the difference of performance.

Example 5

[0111] Inspection Apparatus 2

[0112] The cells of the fuel cells 1 to 3 were used in this example, inwhich a direct methanol fuel cell comprising 10 cells with an electrodearea of 50 cm² were used by connecting in series. An aqueous methanolsolution with a concentration of 2M was introduced at the anode side ofeach cell at a flow rate of 0.6 ml/minute. Air was introduced into thecathode side of each cell at a flow rate of 2000 ml/minutes. The flowrate of air was adjusted using a commercially available mass flowcontroller. I was adjusted to 50 mA/cm², ΔI was adjusted to 5 mA/cm²,T1_(min) was adjusted to 0.5, and T1_(max) was adjusted to 3. All thecurrent flowing in 10 cells was changed from 50 mA/cm²to 55 mA/cm².Voltage changes of the sixth cell (abbreviated as cell 6 hereinafter)and eighth cell (abbreviated as cell 8 hereinafter) were sensed. Acommercially available light bulb was used for the indicator, and thelight bulb was adjusted so that it blinks when T1 is out of theprescribed range. Other indicators available include a buzzer or ring, aLED or lamp, a vibrator, or a smelling device, or a combination thereof.The indicator is not always required.

[0113]FIG. 17 shows the time dependent change of the voltage. The cell 6was decided to be defective, while the cell was decided to be good fromthese results. Referring to the I-V curve, it was shown that theperformance of cell 6 was poor as compared with cell 8 as was indicatedby the inspection results. The result is shown in FIG. 18. While FIG. 19shows the current density dependency of the output current density, themaximum output current density is largely different between cell 8 andcell 6.

[0114] While the catalyst was used by being supported on the carbonblack supports, the catalyst may be supported on other supports such astitanium oxide, may be used without being supported on any supports.While Nafion 20092 made by DuPont Co. was used as the proton conductiveelectrolyte, examples of the electrolytes available in the inventioninclude other perfluorocarbon sulfonic acids (a membrane made by DowChemical Co., Aciplex made by Asahi Chemical Co., and Flemion made byAsahi Glass Co.), sulfonated trifluorostyrene polymer, graftpolymerization electrolytes prepared by introducing sukfonatedpolystyrene graft side chains into a ETFE·FEP base material, sulfonatedstyrene-butadiene random block copolymer, acid dope polybenzimidazole,sulfonated heat resistant polymers (sulfonated polyetherether ketone,polyether sulfone, polyphenyl quinoxalene, polybenzimidazole, andfluorinated polyimide), and ion conductive resin ion containingconductive vinyl monomers (sodium vinylsulfonate, sodium alsulfonate,2-acrylamide-2-methylpropane sulfonic acid). The present invention isalso effective in the fuel cell generator in which other fuels such asethanol, diethylether, dimethoxymethane, formaldehyde, formic acid,methyl formate, methyl orthoformate, trioxane, 1-propanol, 2-propanol,3-propanol, ethyleneglycol, glyoxal, glycerin and aqueous solutionsthereof are introduced to the anode side. The inspection method andinspection apparatus of the invention, and the cell comprising theinspection method of the invention are effective not only in the fuelcell generator, but also in secondary batteries such as a nickelhydrogen secondary battery comprising a hydrogen occlusion electrodemainly comprising a hydrogen occlusion alloy for electrochemicallyoccluding and discharging hydrogen and a nickel electrode mainlycomprising nickel; and a lithium ion secondary battery comprisingpositive and negative electrodes that irreversibly occlude and dischargelithium ions, and an organic electrolyte solution in which anelectrolyte containing lithium ions are dissolved, while the positiveelectrode and negative electrode are disposed with interposition of aseparator.

[0115] As described above, the inspection method and inspectionapparatus of the invention enable simple and objective inspections ofthe performance and transient response of the fuel cell. The direct fuelcell generator of the invention permits a generator comprising aninspection apparatus for deciding the performance of the cell and beingcontrolled with high accuracy to be provided.

What is claimed is:
 1. An inspection method for a direct liquid fuelcell generator having a plurality of cells, each comprising an anodeelectrode including an anode catalysis layer, a cathode electrodeincluding a cathode catalyst layer, and an electrolyte disposed betweenthe anode electrode and the cathode electrode, for power generation bysupplying a liquid fuel to the anode electrode and an oxidant gas to thecathode electrode, wherein validity of the fuel cell is decided based onan observation result of time dependent changes of the voltage V of onecell which is caused by generating a current density change ΔI or −ΔI(ΔI in mA/cm² unit represents a positive value) satisfying the conditionof 0.2≦ΔI≦10 in a current density I (mA/cm²), which is taken out from anarbitrary number of cells connected in series constituting the directliquid fuel cell generator under power generation, during a timeinterval Δt (sec) satisfying the condition of 10⁻⁵≦Δt≦0.5.
 2. Theinspection method for a direct liquid fuel cell generator according toclaim 1, wherein the current density I (mA/cm²), which is taken out froman arbitrary number of cells connected in series constituting the directliquid fuel cell generator under power generation, is changed in therange of 0.2≦ΔI≦5 during the time interval Δt (sec) satisfying thecondition of 10⁻⁵≦Δt≦0.5.
 3. The inspection method for a direct liquidfuel cell generator according to claim 1, wherein the current density I(mA/cm²), which is taken out from an arbitrary number of cells connectedin series constituting the direct liquid fuel cell generator under powergeneration, is changed in the range of 0.2≦ΔI≦2 during the time intervalΔt (sec) satisfying the condition of 10⁻⁵≦Δt≦0.5.
 4. The inspectionmethod for a direct liquid fuel cell generator according to claim 1,wherein the liquid fuel is an aqueous solution of at least one oforganic compounds selected from ethanol, diethylether, dimethoxyethane,formaldehyde, formic acid, methyl formate, methyl othoformate, trioxane,1-propanol, 2-propanol, 3-propanol, ethyleneglycol, glyoxal andglycerin.
 5. The inspection method for a direct liquid fuel cellgenerator according to claim 4, wherein the liquid fuel is an aqueousmethanol solution.
 6. The inspection method for a direct liquid fuelcell generator according to claim 1, wherein the fuel cell is inspectedbased on an inspection criterion that the lapse of time of a change ΔIin the current density I from an initiation time to a time when thevoltage V becomes minimum, or the lapse of time of a change −ΔI in thecurrent density I from an initiation time to a time when the voltage Vbecomes maximum, is within a previously determined time interval.
 7. Aninspection apparatus for a direct liquid fuel cell generator having aplurality of cells, each comprising an anode electrode including ananode catalyst layer, a cathode electrode including a cathode catalystlayer, and an electrolyte disposed between the anode electrode and thecathode electrode, for power generation by supplying a liquid fuel tothe anode electrode and an oxidant gas to the cathode electrode, theapparatus comprising: a load connected to an output from the directliquid fuel cell generator to consume output power thereof; meansconnected to the output from the direct liquid fuel cell generator, forchanging an output current density by controlling the load; means formeasuring an output voltage from the direct liquid fuel cell generator;and a decision device connected to the current density control means andvoltage detection means, for discriminating the condition of the fuelcell generator from the initiation time of the current density change,and from the measured results of the change of the output voltage. 8.The inspection apparatus for a direct liquid fuel cell generatoraccording to claim 7, wherein the load connected to the direct liquidfuel cell generator comprises at least two loads, i.e., an external loadfor stationary consumption of the output electric power from the directliquid fuel cell generator, and an inspection load for generating acurrent density change.
 9. The inspection apparatus for the directliquid fuel cell generator according to claim 7, wherein the liquid fuelis an aqueous solution of at least one of organic compounds selectedfrom ethanol, diethylether, dimethoxyethane, formaldehyde, formic acid,methyl formate, methyl othoformate, trioxane, 1-propanol, 2-propanol,3-propanol, ethyleneglycol, glyoxal and glycerin.
 10. The inspectionapparatus for the direct liquid fuel cell generator according to claim9, wherein the liquid fuel is an aqueous methanol solution.
 11. A directliquid fuel cell generator having a plurality of cells, each comprisingan anode electrode including an anode catalysis layer, a cathodeelectrode including a cathode catalyst layer, and an electrolytedisposed between the anode electrode and the cathode electrode, forpower generation by supplying a liquid fuel to the anode electrode andan oxidant gas to the cathode electrode, the liquid fuel cell generatorcomprising: a load connected to an output from the direct liquid fuelcell generator to consume output power thereof; means connected to theoutput from the direct liquid fuel cell generator, for changing anoutput current density by controlling the load; means for measuring anoutput voltage from the direct liquid fuel cell generator; and adecision device connected to the current density control means andvoltage detection means, for discriminating the condition of the fuelcell generator from the initiation time of the current density change,and from the measured results of the change of the output voltage. 12.The direct liquid fuel cell generator according to claim 11, wherein theload connected to the direct liquid fuel cell generator comprises atleast two loads, i.e., an external load for stationary consumption ofthe output electric power from the direct liquid fuel cell generator,and an inspection load for generating a current density change.
 13. Thedirect liquid fuel cell generator according to claim 10, wherein theliquid fuel is an aqueous solution of at least one of organic compoundsselected from ethanol, diethylether, dimethoxyethane, formaldehyde,formic acid, methyl formate, methyl othoformate, trioxane, 1-propanol,2-propanol, 3-propanol, ethyleneglycol, glyoxal and glycerin.
 14. Thedirect liquid fuel cell generator according to claim 13, wherein theliquid fuel is an aqueous methanol solution.